Long life high pressure lamp



p 194-3. 6. A. FREEMAN 2,330,042

LONG LIFE HIGH PRESSURE LAMP Filed Aug. 21, 1941 INVENTOR G. A. F FEM/7N ATTORNEY Patented Sept. 21, 1943 UNITED STAT LONG LIFE HIGH PRESSURE LAMP George A. Freeman, East Orange, N. 3., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa, a corporation of Pennsylvania Application August 21, 1941, Serial No. 407,731

4 Claims. (Cl. 176-122) This invention relates to electric devices of the gaseous conduction type and more particularly to high pressure gaseous conduction devices of which illustrative examples are found in Letters Patent Nos. 2,241,345 and 2,241,362, owned by the assignee of the present application.

In the patented devices and similar devices known in the art, a discharge is started between unheated electrodes, and in order to support this discharge an ionizable medium, such as mercury or the like, is employed, together with a rare gas to facilitate starting of the discharge, the latter taking place in an envelope made of quartz, or of glass with a high silica content so that the envelope may withstand the operating temperatures of the lamp, which are desirably of the order of from 500 C. to 900 C.

By concentrating power into small space in a quartz mercury lamp, higher temperatures are employable and higher initial efliciencies are obtained, and the smaller the quartz bulb, the lower will be the cost of the lamp. It has been discovered by" the present applicant that with a given size of quartz bulb the wattage may be increased to an unusual extent in an air cooled lamp, and that such a quartz mercury lamp may be operated with good results at higher wattages per unit area of envelope than other workers have deemed practical, such operation being, for example, in contradistinction to the evaluation of such lamps suggested in Letters Patent to Ellenbaas No. 2,135,702, and comparable favorably with the results stated by that patentee to be predictable for devices made according to the directions contained in the patent.

In that patent a formula is given according to which when the potential across a tubular mercury vapor arc discharge device is limited by a ballasting device to a certain voltage, and the power consumption in the discharge tube is 250 watts, if the inside diameter of said discharge device be 1.5 cm., and the length of the discharge path within the device be 4 cm., a long useful life may be expected if the said lamp be connected to a 220 volt source of current of commercial frequency, in circuit with a choke coil (as the ballasting device) the burning voltage being 120 volts in the case of normal operation after equilibrium is attained.

It is also stated that the two thermionic electrodes of the last-mentioned lamp consist of coilshaped tungsten wires which are coated with an earth alkali oxide, and the discharge bulb is stated to be made of quartz, filled with argon,

and a small quantity of mercury, having at room temperature a pressure of 10 mm.

It is further stated that the space between the discharge bulb of this patent and the surrounding container bulb is nitrogen-filled and has, at room temperature, a pressure of approximately 50 cm., greatly in excess of the pressure within the quartz discharge bulb.

The above statements in the patent to Elenbaas may desirably .be supplemented bythe following calculation as to the wattage made effective per unit of radiating area under the conditions set forth by said patent, in a lamp having a cylindrical quartz bulb with an internal diameter of about 1.5 cm. and a' discharge are 4.0 cm. long,

The radiating area is the cylinder of area thus increasing the illuminating efiiciency of the lamp to a substantial extent.

The above is cited as a typical example of a quartz mercury discharge lamp in which the thermionic electrodes, are activated, having a coating of an alkaline earth compound; while it is to be noted that nitrogen is used in the outer space at a high pressure differential relatively to the mercury and argon used as the ionizable medium between the thermionic electrodes in the quartz bulb.

However, in the course of research originally intended to increase the efliciency at high,

wattages of air-cooled lamps, tests were made by the present applicant, of quartz bulbs fitted with thermionic electrodes activated by an alkaline earth compound and acting in a space comparable to that of the lamp described in the foregoing paragraphs, and operating with an applied wattage of 400, with the quartz tube surrounded by a space filled with nitrogen at a pressure higher than that of the argon in the discharge tube, and operating at a temperature considerably in excess of 800 C. These tests resulted in a softening of the quartz bulbs enough to cause them to swell slightly after a hundred or more hours of operation and it is known that at such temperatures quartz deteriorates rapidly by chemical and physical action if alkaline materials are present.

Indeed, even with no chemical action on the highly heated quartz there is a slight frosting of the quartz by virtue of the devitrificatipn attending the elevated temperatures alone. The ensuing roughening is of no serious consequence of itself, since the bulb wall remains transparent, but the roughness so developed on the inner surface of the quartz bulb may hold more firmly any particles sputtered from the electrodes, with consequent darkening of the envelope and loss of lighting efficiency.

The lamps constructed and tested as above, developed a further disadvantage, in the important respect that all of the lamps which had their outer envelopes filled with nitrogen became inoperative after burning for a period of 200 to 500 hours.

Accordingly, it has been the primary object of the present invention to provide a gaseous discharge tube which shall be free from the disadvantages and inconveniences encountered in practical experience with the earlier structures as above enumerated, while retaining the features of proved advantage, and to provide improvements in structure and operation which shall afford more reliable performance, increased life and greatly enhanced lighting efiiciency and economy, due to the improved capacity of the lamp to operate under sustained higher wattages.

In pursuance of the above primary object, an? other object is to adapt for use in the lamp of the present invention a form of thermionic electrode free of alkaline materials and preferably providing the favorable characteristics of a nonactivated thermionic metal of the group which includes thorium, uranium and similar metals, substantially in accordance with the disclosure in the above-mentioned Patents Nos. 2,241,345 and 2,241,362, of which I am a joint inventor, and by such provision I have found that it is possible to prevent sputtering from the electrodes and attendant darkening of the quartz envelope.

Another object of the instant invention is to provide in the space between the quartz bulb and the outer bulb an environment free from nitrogen, and from other gases which will pass through the walls of the quartz tube at fairly rapid rates when the tube is operated at near softening temperatures, examples of such gases being hydrogen, helium, neon and others of the lighter gases, it having been found by me, as the result of prolonged and careful analysis of the above mentioned tests during practical experience of the life history of the aforesaid tested lamps, that the curtailed life of such lamps is due largely, if not entirely, to the contamination of the arc chamber because of its invasion by nitrogen in the manner above designated.

I have further discovered that this contamination and its deleterious effects may be wholly avoided by the provision of a substantial degree of vacuum in the outer bulb of such lamps, eliminating the use of nitrogen and its congeners, and providing instead an evacuated environment for the quartz tube which is free from any element capable of contaminating the arc space within the quartz tube; or by providing an attenuated environment in the outer bulb constituted by the introduction into that space of a suitably inert gas, at a suitable pressure which in operation substantially balances the external pressure of the atmosphere. Accordingly I may use argon in the outer space or one of the other desirably heavier gases, such as krypton, xenon, or mixtures thereof, the use of argon being favorable at present by reason of its relatively lower cost.

If traces of argon escape into the outer bulb, no harm will be done, and I have recently found by experiment and by prolonged tests that the life of a 400 watt lamp of my improved type above exemplified will be extended from a maximum of 5000 hours to a minimum of 1500 hours by merely leaving out the customary nitrogen from the outer bulb and using a vacuum in the space.

In fact, the indications are that not only in the quartz tubes of the gaseous discharge lamps herein exemplified is it possible to attain such prolongation of the life of the lamps, but also that it is practicable by a similar application of the present invention to secure an optimum commercial life of 5000 or more hours in any quartz lamp.

Still further features and objects of the present invention will become clear to those skilled in the art by reference to the single figure of the accompanying drawing, wherein is illustrated in front elevation, partly in section, a high pressure gaseous discharge device in the construction of which the present invention has been embodied.

Referring now to the drawing in detail, a gaseous discharge device of the high pressure mercury type is shown which comprises an envelope l of quartz, or of hard glas with a high melting point, so as to withstand the operating temperatures of the lamp. A pair of thermionic electrodes 2 and 3 are disposed therein at opposite ends of the envelope and are supported by leading-in conductors or exterior terminals 4 and 5 preferably made of tungsten or like refractory metal.

After exhaustion of the envelope I, it is filled with an ionizable medium, such as mercury vapor, of just sufficient quantity to be completely vaporized with a pressure of the order of onehalf to several hundred atmospheres during operation of the device, and in addition a small quantity of a rare heavy gas is preferably introduced, to facilitate starting, for example, such a gas as argon, at an optimum pressure of from 25 to 35 mm.

The two electrodes 2 and 3 may be of identical construction, and each is shown to comprise a core, as 6 and I, made of a suitable non-activated thermionically active metalsuch as thorium or uranium, or metals having similar high melting and vaporization points and thermionic properties, as distinguished from metallic oxides or metallic components of the alkaline earths, the electrodes of the instant invention being designedly free from alkaline materials, for the reasons hereinbefcre set forth.

Preferably this core of high electron emissive metal is covered partially by a material such as tungsten or other refractory metal 8, having a lower electron emissivity than that of the core, at the operating temperatures of the device, and the aforesaid covering of refractory metal, as shown, is preferably in the form of a helix with adjacent turns slightly spaced from each other so as to provide openings for the egress of electrons from the core of highly emissive metal.

Due to its characteristics, the core of thorium not only has a high electron emissivity at the operating temperatures of the lamp, but in addition it is protected by the surrounding shield 8 of tungsten, which assumes the ensuing positive ion bombardment so thatno sputtering of the thorium occurs, which would otherwise afiect deleteriously theenvelope, with a resulting decrease in the efiiciency of both the visible and the invisible radiations generated, inasmuch as such radiations would be absorbed by the envelope instead of being transmitted therethrough. Moreover, since the corematerial is inert with respect to silica, it has no chemicalor physical efiect thereon, and a quartz or other hard glass envelope may be used without frosting or devitrification, and without darkening of the envelope, even if there should be a slight sputtering of the thorium, which is substantially eliminated. The above, and other features of the thorium core and its associated parts, form the subject of generic claims in the aforesaid Letters Patent Nos. 2,241,345 and 2,241,362, to which patents reference may be had for further details of the thermionic electrode structure.

The energizing current is derived by the electrodes 2 and 3 through their exterior terminals 4 and 5 from connections to the conductors 9 and I leading from the customary screw base H through the usual press l2 and thence to supporting uprights l3 and M. The upright 13 is made of conductive material connected electrically, as at IE, to the exterior terminal of the electrode 3. The terminal 4 of the electrode 2 is connected, as at It, to a cross-piece ll of the upright frame 14, the latter being preferably made of nickel, a nickel alloy, or iron, and provided near its upper end with spacing members 18 having parts (not shown) which extend laterally against the walls of the outer bulb 20, acting to hold' the frame I4 in its proper central position. Similar spacing members l9 perform a like duty for the quartz tube near its top and bottom, and one post of the frame I4 i connected, as at 2|, 22, to the conductor 9, to which the other upright 23 is also connected, as by a bend 24.

At 25, a strip 26 is connected electrically to the bend 24 and leads to one end of a resistor 21, which may be of the order of 10,000 ohms, and from the other end of said resistor a like strip 28 leads to an electrical connection 29 with an auxiliary electrode 30 which is constructed and arranged to impart a glow discharge between it and the thermionic electrode 3 in known fashion, to aid in striking the are between the electrodes 2 and 3'.

A vacuum is provided in the space 3| between the quartz tube l and the outer bulb 20. Lamps so constructed have yielded a useful commerial life of from 3000 hours and upward, operating with greatly increased lighting efficiency.

It thus becomes clear to those skilled in the art that a gaseous discharge device of the high pressure mercury type is herein provided which is free from alkaline elements and in which electrodes of thermionic metal are utilized, so constructed and arranged that sputtering from the electrodes, with attendant frosting, devitrification, darkening, and other deleterious effects upon the inner walls of the envelope, are eliminated; also that,

in combination with this improvement in the electrodes, the invention provides for elimination of the heretofore unrecognized source of contamination of the arc space in the quartztube,

customary filling of said space by nitrogen or anothervgas which tends to diffuse through the walls of the quartz tube when the latter is heated to near softening; 'also'that provision is made by the improved structure herein disclosed for a notable increase in the useful life not only of the gaseous discharge device as disclosed herein, but also of other high-temperature lamps using quartz tubes; and finally provision is made for effecting the original construction of such tubes with unusual economy in materials, increasing to an optimum extent the capacity of the quartz bulbs to operate at very high wattages and internal pressures, the applied loading being .increased up to slightly over 20 watts per sq. cm. of internal wall area surrounding'thearc.

Although I have shown and described only one specific embodiment of the invention, '1 do not desire to be limited thereto, as various modifications of the same may be made without departing from the spirit and scope of the appended claims.

I claim:

1 A high pressure discharge lamp comprising an envelope capable of withstanding high operating temperature and pressure and provided;. with an ionizable medium therein at a pressureiranging between one-half and several hundred atmospheres during operation of said lamp, a pair of non-activated electrodes disposed in said envelope to eliminate sputtering thereof and blackening of the envelope walls during operation of .said

2. A high pressure discharge lamp comprising.

a quartz envelope capable of withstanding high operating temperature and pressure and provided with an ionizable medium therein at a pressure ranging between one-half and several hundred atmospheres during operation of said lamp, a pair of non-activated electrodes disposed in said envelope to eliminate sputtering thereof and blackening of the envelope walls during operation of said lamp, means for energizing said lamp at a loading of approximately 20 watts per sq. cm.

of envelope area which raises the temperature of said envelope to near its softening point where diffusion of gas therethrough would ordinarily occur, and an outer bulb surrounding said envelope provided with an inert gas having an atomic weight greater than 39.91 to prevent contamination of the ionizable medium within the latter and shortening of lamp life by difiusion of said inert gas through the softened walls of said envelope.

3. A high pressure discharge lamp comprising an enclosing'quartz envelope provided with an ionizable medium therein at'a pressure ranging between one-half and several hundred atmospheres during operation of said lamp, a pair of non-activated electrodes disposed in said envelope to eliminate sputtering thereof with attendant blackening of the envelope walls and between which an arc discharge occurs during operation of said lamp, an auxiliary electrode mounted in proximity to one electrode of said pair and operable to initiate a discharge between said pair of non-activated electrodes; a resistor connected to said auxiliary electrode and the most remote electrode of said pair to ballast, the starting circuit for said lamp, means for energizing said lamp at a loading sufficient to raise the temperature of said envelope to approximately its softening point where diffusion of gas therethrough would ordinarily occur, and an outer bulb surrounding said envelope and provided with an environment having an atomic weight greater than 39.91 to render the same non-diffusible through the heated walls of said envelope to prevent contamination of the ionizable medium within the latter and shortening of lamp life.

4. A high pressure discharge lamp comprising a cylindrical enclosing quartz envelope provided with an ionizable medium therein at a pressure ranging between one-half and several hundred atmospheres during operation of said lamp, a pair of non-activated electrodes disposed in said envelope to eliminate sputtering thereof with blackening of the envelope walls and between which an arc discharge occurs during operation of said lamp, means for energizing said lamp at a loading of approximately 20 watts per sq. cm. of envelope area to raise the temperature of said envelope to approximately 800 C. where diffusion of gas therethrough would ordinarily occur, an outer bulb surrounding said envelope and provided with an environment having an atomic weight greater than 39.91 to render the same non-difiusible through the heated walls of said envelope to prevent contamination of the ionizable medium within the latter and shortening of lamp life, and said envelope having a radiating area calculated according to the formula:

wherein d is the internal diameter of the cylindrical envelope and e the length of the arc discharge between the electrodes.

GEORGE A FREEMAN. 

